Many processes for preparing aliphatic alcohols, alicyclic alcohols or aromatic alcohols by hydrogenating carboxylic acids or carboxylic acid esters have been proposed since the 1930s.
Copper base catalysts have been mainly proposed as the catalyst for hydrogenation of carboxylic acid esters, especially fatty acid esters, and in general, a copper-chromium catalyst is preferably used on an industrial scale. However, the copper-chromium catalyst involves serious problems in connection with the disposal of waste water discharged from the catalyst-preparing process or the used catalyst, though the copper-chromium catalyst is excellent in catalyst performance. In the preparation of higher aliphatic alcohols where drastic reaction conditions are required for hydrogenation of carboxylic acid esters, and since high-temperature and high-pressure conditions such as a temperature of 250.degree. to 300.degree. C. and a pressure of 200 to 300 atomospheres are necessary for the preparation of higher aliphatic alcohols, there are various problems concerning equipment investment and maintenance. Moreover, since the reaction is carried out at a high temperature, formation of a side reaction product cannot be avoided, and use of this catalyst is not preferred from the energy-saving viewpoint.
Accordingly, development of a technique capable of hydrogenating a carboxylic acid ester at a low temperature or under a low pressure has been desired.
Attempts have been made to hydrogenate higher fatty acid esters at a low temperature or under a low pressure, but in most of these attempts, the intended object is attained by using a large quantity of a copper-chromium catalyst.
Adkins et al. report that in hydrogenating methyl laurate by the liquid-phase suspended bed reaction, lauryl alcohol is obtained at such a low temperature as 150.degree. C. by using such a large amount of a copper-chromium catalyst such as 150 parts by weight per 100 parts by weight of the ester [J. Amer. Chem. Soc., 70, 3121 (1948)]. In this process, such a high hydrogen pressure as 340 atmospheres is adopted, and the reaction time is long though a large amount of the catalyst is used. Ueno et al. report that in hydrogenation of various fatty acid esters, aliphatic alcohols are obtained under low hydrogen pressure such as 10 to 25 atmospheres (initial charge pressure at room temperature) by using 10 parts by weight of a copper-chromium catalyst per 100 parts by weight of the ester [Kogyo Kagaku Zasshi, 38, 1105 (1935); ibid, 48,18 (1945)]. However, the reaction temperature is as high as 270.degree. to 300.degree. C., and the amount of unsaponified product formed by side reaction is large while the yield of the intended alcohol is very low and only 30 to 60%.
Japanese Patent Laid-Open No. 38333/1985 discloses an attempt to hydrogenate methyl laurate under reaction conditions of 218.degree. C. and 60 bars by using copper-chromium as one component of the catalyst and adopting the fixed bed reaction method, but this process is defective in that the liquid hourly space velocity (LHSV) is 0.2 and productivity is very low.
Separately, catalysts not containing harmful chromium have been developed. For example, according to Japanese Patent Laid-Open No. 56139/1986, a higher fatty acid ester is hydrogenated under such a low pressure as 50 bars in the presence of a nickel catalyst by the fixed bed reaction. However, the reaction temperature is as high as 250.degree. C. and the yield of the alcohol is very low and below 40%.
As is apparent from the foregoing description, according to any of the conventional techniques using a copper-chromium catalyst or a catalyst not containing harmful chromium, it is impossible to prepare a higher alcohol under low-temperature and low-pressure conditions by hydrogenation of a higher fatty acid ester while maintaining high productivity and high selectivity.